Fossil Seawater affects Coastal Aquifer

Rising
salinity in the primary source for desalinated tap water in North Carolina’s Outer Banks has been traced to fossil seawater and not,
as some feared, to seawater intrusion. Duke University researcher Avner Vengosh
says the saline groundwater in the Yorktown aquifer can remain a valuable
source of desalination for decades to come, without having to switch to the costly
alternative of desalinating seawater.

Vengosh, professor of
geochemistry and water quality at Duke’s NicholasSchool of the Environment, recently directed a study to
measure and analysesalinity levels
in the Yorktown aquifer and identify their source. The study
appears in the on-line version of Hydrogeology Journal, a peer-reviewed
publication.

Salinity levels
in the aquifer are roughly two-and-a-half times higher today than when the Dare
County North Reverse Osmosis Water Plant in Kill Devil Hills began pumping and
desalinating groundwater in the late 1980s. Some feared the rise was caused by
seawater seeping into the aquifer. However, by using geochemical and boron
isotope tracers, Vengosh’s research team found that the increase is from an
upflow of old and diluted seawater which was trapped long ago in the Atlantic
coastal aquifers.

That can be viewed as
good news, Vengosh explains. “As more and more water is pumped out of the Yorktown aquifer to meet
growing year-round demand, the groundwater level is dropping and the relative
proportion of fossil seawater, flowing up from deeper aquifers, is increasing,”
he says. “As fossil seawater mixes with the remaining fresh groundwater, it is
raisingsalinity but
at a relatively slow and steady rate that is more manageable and sustainable
than the rapid increase we’d see if there was modern-day seawater intrusion.”

Tests showed that the DareCounty water plant’s reverse osmosis membranes still remove about 96–99% of
the dissolved salts from the aquifer’s groundwater. The membranes haven’t
remained as effective at removing boron and arsenic, which occur naturally in
deep saline groundwater. Tests of the water plant’s four wells found the
membranes remove only between 16–42% of the boron in the water and 54–75% of
the arsenic. The arsenic levels (which are below safe drinking levels after
additional treatment that
follows the reverse osmosisdesalination) aren’t expected to rise in
coming decades, but boron levels likely will as the salinity of
the aquifer rises.

“Boron isn’t currently
regulated as a drinking water contaminant in the United States, but there are
international recommendations about safe levels for human consumption,” Vengosh
says. “Additional treatment might
be needed to remove boron from the desalinated DareCounty groundwater.”

Even with the
additional costs, desalinated groundwater remains a bargain compared to
desalinating seawater, Vengosh says. Desalinating seawater requires substantial
additional capital investments and advanced filtrationtechnology, largely because of the
quantity of salts that must be removed. Seawater contains about 35 ppt of dissolved
salt. The groundwater currently used by DareCounty has a dissolved salt
content of about 5 ppt.

“Given a choice,
groundwater desalination is
the way to go, as long as you take care of other contaminants such as arsenic,”
Vengosh says.

The new study is the
first to directly link fossil seawater to rising salinity in
a groundwater aquifer. “It is intriguing that the coastal aquifers in the
southeastern US contain a large volume of brackish water that could sustainably
be used for desalination or
any other applications that can tolerate their relatively low salinity, particularly as other water sources
are at risks due to climatic change and human stress,” he says. “The
implications of this study may extend far beyond the Outer Banks.”

The lead author of this
paper is doctoral student David S. Vinson who will graduate this spring. Other
co-authors were senior research scientist Gary S. Dwyer from Duke’s Nicholas
School and former undergraduate Haylee G. Schwartz, who earned a Bachelor of
Arts in Earth and Ocean Sciences from Duke in 2009 and is now at the UCLA
School of Law. Schwartz’s participation in the study was funded by a grant from
the Duke University Undergraduate Research Support Office.